1. Technical Field
This disclosure relates to an inter-connector that is interposed between two serially connected unit cells to provide mechanical strength to and to conductively couple a serial connection between unit cells.
2. Discussion of Related Art
In general, alkali storage batteries—such as nickel-hydride storage batteries, nickel-cadmium storage batteries, etc. or lithium-based storage batteries such as lithium-ion cell, lithium-polymer cell, etc.—are generally manufactured by interposing separators between cathodes and anodes, then helically winding them; coupling current collectors to the ends of the cathodes and the anodes to form electrodes, disposing the electrodes within metallic outer cases, welding lead portions extending from the current collectors to seals; and mounting the seals on openings of the outer cases while interposing insulating gaskets therebetween. Because such an alkali storage battery requires a high output when used for example, as a power source for an electric motor tool or for an electric automobile, etc., storage batteries, have generally been assembled into module cells made by connecting a plurality of individual cells in series. Where storage batteries are used in high output applications, such as powering a large electric motor or an electric automobile, the module cell comprising a plurality of cylindrical unit cells coupled in serial and/or in parallel have been used.
FIG. 1 shows in perspective a plane-frame module cell 10 comprising a parallel arrangement of cylindrical serial cells 30 in a frame 20, the cylindrical serial cells 30 being formed by serially connecting a predetermined number of the cylindrical unit cells 40. Although only a lower frame receiving the cylindrical serial cells 30 is shown in the FIG. 1, there may also be one or more upper frames having structure similar to that of the lower frame stacked therein.
In the cylindrical serial cells 30 shown, a serial inter-connector is interposed between the two serial unit cells 40, forming a serial interface that mechanically and electrically couples the two unit cells 40. When coupling unit cells 30 using an inter-connector, the inter-connector is typically welded to the cathode terminal surface of one unit cell 40 and the outer wall of the other unit cell 40.
Contact resistance welding is commonly used in coupling the inter-connector to the cells because it is useful for welding small objects. Contact resistance welding has some drawbacks, however. To obtain a good quality weld, a welding electrode is preferably contacted with the inter-connector during the welding process. In the serial interface between unit cells 40, welds to the cathode terminal surface or the outer wall of the unit cell are formed from melted portions of the inter-connector. The inter-connector should have sufficient thickness to securely hold the two unit cells together. If the inter-connector is thicker than the outer wall of the unit cell, the current from a welding electrode contacting the inter-connector flows through the inter-connector instead of to the outer wall of the unit cell. Consequently, at best, a poor weld is formed between the inter-connector and unit cell.
Typically the outer wall of a unit cell is not very thick, for example, from about 0.4 T to about 0.5 T. In general, the side wall of the unit cell is about 0.4 T thick and the bottom is about 0.5 T thick. Here, 1 T corresponds to about 1 mm. An inter-connector thinner than the unit-cell outer wall may not have sufficient strength to secure the two unit cells unless expensive metal materials are used in the inter-connector.